Light emitting diodes (LEDs) are an important class of solid-state devices that convert electric energy to light. Improvements in these devices have resulted in their use in light fixtures designed to replace conventional incandescent and fluorescent light sources. The LEDs have significantly longer lifetimes and, in some cases, significantly higher efficiency for converting electric energy to light. LED-based white light sources are typically made by packaging one or more blue LED chips with suitable yellow and red phosphors.
For the purposes of this discussion, an LED chip can be viewed as a semiconductor having three layers, the active layer sandwiched between two other layers. The active layer emits light when holes and electrons from the outer layers combine in the active layer. The holes and electrons are generated by passing a current through the LED chip. The LED chip is powered through an electrode that overlies the top layer and a contact that provides an electrical connection to the bottom layer.
The cost of LED chips and their power conversion efficiency are important factors in determining the rate at which this new technology will replace conventional light sources and be utilized in high power applications. The power conversion efficiency of an LED chip is defined to be the ratio of optical power emitted by the LED chip in the desired region of the optical spectrum to the electrical power dissipated by the light source. Electrical power that is not converted to light that leaves the LED is converted to heat that raises the temperature of the LED. Rise in the chip temperature places a limit on the power level at which an LED operates. In addition, the conversion efficiency of the LED generally decreases with increasing current especially at the higher current densities that enable lowering the cost of light; hence, while increasing the light output of an LED by increasing the current increases the total light output, the electrical conversion efficiency is decreased by this strategy. In addition, the lifetime of the LED is also decreased by operation at high currents.
LED light sources made from a single LED chip even as large as a square millimeter in size, are not yet capable of generating sufficient light to replace conventional light sources for many applications. In general, there is a limit to the light per unit area of LED that can be practically generated at an acceptable power conversion efficiency. This limit is imposed by the power dissipation and the electrical conversion efficiency of the LED material system. Hence, to provide a higher intensity single LED source, larger area chips must be utilized; however, the light extraction efficiency reduces as chip size gets bigger for most types of LED chips and also there is a limit to the size of a single LED chip that is imposed by the fabrication process used to make the LED chips. As the chip size increases, the yield of chips due to random defects decreases, and hence, the cost per LED chip increases faster than the increase in light output once the chip size increases beyond a predetermined size.
Hence, for many applications, an LED-based light source must utilize multiple LEDs to provide the desired light output. For example, to replace a 100-watt incandescent bulb for use in conventional lighting applications, approximately 25 LED chips of the order of 1 mm2 size are required. This number can vary depending on the color temperature desired and the exact size of the chips. The drive voltage for a typical GaN LED chip is typically about 3.2-3.6V. If all of the LED chips are connected in parallel, the DC power supply must deliver a large current at a low voltage, which presents challenges in terms of AC to DC power conversion efficiency and the size of the conductors that must be used to deliver the high currents without dissipating a significant fraction of the power in resistive losses.
One method for reducing these problems involves dividing a die of more or less optimum size into a plurality of series connected LED segments. Such a structure is shown in co-pending application Ser. No. 12/208,502, filed on Sep. 11, 2008, which is hereby incorporated by reference. The optimum size of a die depends on the details of the chip design and on the yield of the semiconductor process used to fabricate the dies. For any given process there is an optimum size from a cost point of view. If the die is used as a single LED with a drive voltage of the order of 3 volts, a large current must be provided at the die to maximize the light output. If the die is divided into N smaller LED segments that are connected in series, the drive voltage is increased by a factor of N, and the drive current is decreased by a factor of N, which provides improvements both in the efficiency of the power supply that provides the drive current and a reduction in the resistive losses within the die.
One prior art method for dividing the die into the component LED segments involves cutting isolation trenches that extend from the surface of the die to the underlying resistive substrate to isolate the individual component LEDs from one another. The individual component LEDs are then connected in series by providing a conductor that connects the n-layer of each component LED to the p-layer of an adjacent component LED. These deep trenches increase the cost of production of the dies and interfere with the extraction of light from the sides of the die.